ok
1. watts per gallon is a good rule of thumb, but not entirely effective in reef tanks because it does not account for the light at the bottom of your tank. your light at the top of the water surface is going to be the most intense and while you will still have 4 wpg, you will not have intense light at the bottom of your tnak, where many reefer place clams and such.
2. 3-5 wpg is terribly low, and will not keep sps or clams alive. if you are going to go by th ewatts per gallon rule then try to shoot for anywhere between 6-10 watts per gallon if you want to keep sps and clams.
3. a 175 mh, will be very intense up to 18 inches deep , a 250 is good for 18-24 inches deep and a 400 is good 24 to 36 inches deep.
if you want intense light on your snadbed, and hope to keep clams there, i think you should be ok, with the 2 250 watt mh's, and change all of your existing 96w of pc to 10K bulbs, and then still add 2 110 vhos, or you could even drop down to two 4 ft Ho actinics at 40 watts a piece. i think that this would be ideal for you, but then again it is up to you. 3-5wpg is horrible, and i wouldnt even put shrooms in a tank that was lit like that, but at the same time to each his own. i agree that the 400w mh's would be in excess, but for the depth of your tnak , they would be perfect. i think that if you went with the 250's kept your current pc's and added two actinic ho's you would be settign pretty!
good luck
jon
